1. Field of the Invention
The present invention relates generally to sensors and actuators. More specifically, the present invention relates to piezoelectric composite apparatus and related methods of manufacture and use.
2. Description of the Related Art
Various types of platforms such as, for example, aircraft structural components, aircraft skins, or other related components, when in operation, are subjected to various environmental conditions such as stress and strain, exposure to temperature extremes, and significant vibration energy. Due to the various environmental conditions, such components can suffer material degradation over time.
Structural health monitoring helps promote realization of the full potential of such components. Remotely positioned sensors have been installed adjacent to, placed on, or embedded within such structures/components to monitor various parameters such as, for example, strain levels, stress, temperature, pressure, or vibration level to help manage physical inspection schedules, maintenance schedules, to help predict material failure, and generally monitor the “health” of such components.
Such sensors have been made using a composite material which can generate small electrical currents when the material is deflected, such as when the monitored component is subjected to a stress or strain. Piezoelectric material is but one example of materials that can perform this function. As a stress or strain is applied to the structure being monitored, the body of the sensor deforms or deflects, causing the piezoelectric material to convert a fraction of the mechanical energy of the stress or strain to electrical energy.
Conventional piezoelectric sensors are typically manufactured using a layer of piezoelectric material positioned between either planar or interdigitated electrodes made of, for example, copper, gold, etc., or combination thereof, and positioned in either direct or indirect contact with the piezoelectric material to form a relatively thin laminate. One methodology of forming the sensor includes using piezoelectric fiber composite material. Such piezoelectric material can be made by a process which includes bonding together relatively thin piezoelectric wafers using a liquid epoxy to form a stack, bonding the stack using a relatively moderate pressure and temperature, and slicing the stack to form multiple sheets of piezoelectric fiber composite material having each strand of the piezoelectric material separated by the epoxy.
The sensors can be attached to or embedded within various structures, without damage, to be monitored using, for example, a graphite epoxy composite which can withstand temperatures of up to approximately 150° C. to 160° C. Depending upon the conductive properties of the material to be monitored or of the material used to connect the sensor to the material to be monitored, an insulator such as glass epoxy or a polyimide such as Kaplon® manufactured by DuPont® can be used.
Completed sensors and other apparatus manufactured using conventional piezoelectric composite methodologies and materials have not been able to be embedded within materials such as, for example, carbon or carbon fiber composites, which are subjected to high pressures and high composite laminating temperatures, as part of their own manufacturing, or extreme pressures and temperatures when in operation. Recognized by the inventors is the need to utilize bonding materials, both within the structure of the sensor and to connect the sensor to the structure to be monitored, that are capable of withstanding high-pressure and high temperature such as those present when the sensors are used in structures subjected to high pressures and high temperatures as part of their own manufacturing and sensor embedding process or in operation.
Regardless of the material composition of the structure to be monitored, installation of the sensors adjacent to or within the structure to be monitored has required the user to first determine the orientation of usable or expected strain or other health indicator and then position the sensors so that the electric field generated by the piezoelectric material-electrode combination would be at least somewhat oriented along the axis associated with the health indication. Recognized by the inventors is that this determination cannot always be readily made because the axis of the usable strain can vary over time. Further, recognized by the inventors is that even if the axis of usable strain were fixed in time, the sensors may not always be properly positioned and can often not be properly nondestructively re-positioned, especially where the sensors are embedded within the structure to be monitored. Thus, recognized is the need for a piezoelectric apparatus which can be readily adapted to align the axis of the sensor, electric field with the axis of usable strain to enhance energy transfer.